Crystalline and multicrystalline silicon photovoltaic panels are traditionally fabricated using a semi-automated process that requires expensive manufacturing equipment, is relatively labor-intensive, and requires vacuum processing tools such as vacuum evaporators and plasma enhanced chemical vapor (PECVD) deposition chambers. The invention described herein describes a continuous, roll-to-roll crystalline PV manufacturing method that requires no vacuum tools. Roll-to-roll (R2R) manufacturing of PV panels have been demonstrated using plasma deposited amorphous silicon and slot-die coated copper indium gallium diselenide, but these processes have not been truly continuous, e.g., R2R equipment is used but the web is wound up and transported to multiple process stations. Moreover, the cost of manufacturing per Watt of generated power from these thin film panels has been financially unsustainable.
In the present invention, the PV panels are made of high efficiency crystalline silicon microspheres 10-150 microns in diameter, which greatly reduces silicon consumption per panel area. The PV panel makes very efficient use of the silicon since the light-incident surface area-to-volume ratio is 2-3 orders of magnitude greater than planar silicon. The small sphere size also allows the microspheres to be dispersed in an ink system that is coated on a web into a closed packed monolayer. High throughput, low-cost coating of the microspheres and other functional layers, and formation of the PN junction are all carried out in a continuous, atmospheric pressure roll-to-roll process.
US patent application publication no. US2010/0167441, entitled, Method of Manufacturing a Light Emitting, Photovoltaic or Other Electronic Apparatus and System, is assigned to the present assignee and incorporated herein by reference. The publication describes various techniques to form light emitting diode (LED) sheets and photovoltaic (PV) panels using arrays of semiconductor microdiodes. In particular, the PV panels are comprised of microspheres and may be on the order of 20-40 microns in diameter. Several methods of manufacturing silicon spheres are known and include forming spheres from molten silicon in a drop tower, patterning silicon particle agglomerates on a substrate and melting them to form spheres by surface tension, or dropping powder through a plasma reactor.
To-date, spherical PV modules have been limited by a means to rapidly produce a nearly closed packed monolayer of silicon spheres. Monolayer formation of micrometer or nanometer range spheres has been a significant area of research across a number of different disciplines over the years. Rapid, inline formation of true monolayers of micrometer spheres from a high solids fluid is difficult and, within an industrial setting, has remained a difficult task. Monolayers occur within very narrow control ranges where a small variance in print conditions favor either sparse layers or layer doubling.
Lee et al, US patent application 2011/0117694 A1 describes an inkjet printing process to make silicon microsphere diodes in a monolayer but not in a closed packed array, and inkjet printing is relatively a low-throughput printing process compared to the coating processes described herein. Moreover, the PV panel process uses vacuum tools, specifically plasma enhanced chemical vapor deposition to form the electrodes. What is needed is a high throughput (e.g., 10-20 ft/min) R2R monolayer coating process of silicon microspheres.
Back surface field (BSF) formation in spherical PV diodes at low temperature (<640° C.) is also needed to increase panel efficiency and maintain the structural integrity of the web during a R2R process. A BSF is an aluminum rich region in a silicon solar cell that is capable of providing a 1-3% total power conversion efficiency gain in a solar panel. Typically, the rear contact for mono-crystalline and multi-crystalline silicon solar cells is formed by screen printing an aluminum paste on the back-side of a silicon wafers and firing them at 800-900° C. to form an ohmic contact and a BSF. U.S. patent application Ser. No. 13/587,380 describes an aluminum-based ink. This ink is utilized to form a BSF in silicon microspheres using rapid annealing at a peak temperature of 600° C. on a moving web.
Various methods of doping the silicon spheres to form diodes are also known. Typically, lightly doped p-type silicon (1-10 Ohm-cm) is highly doped (1e-4 Ohm-cm or less) on the outer surface with phosphorus, to form a pn+ diode. U.S. Pat. No. 7,214,577 describes using standard diffusion of phosphorus dopants into 1-2 mm diameter silicon spheres before forming the PV panels. This is a batch process that requires a special process chamber to contain hazardous gas, and later the spherical diodes must be etched to remove a portion of the n+ region. The method described in the present disclosure forms the PN junction in-situ during the R2R process using laser annealing at atmospheric pressure. This is the first time laser annealing is used to form PN junctions on spherical silicon in-line, and it removes the need for etching the diode in later processes.
The anodes and cathodes of the diodes are ohmically connected to printed conductors to form an array of parallel-connected diodes in a PV panel. Panels may be connected in a combination of series and parallel to achieve the desired electrical characteristics.
Further, the panels described in US patent application publication no. US2010/0167441 are formed using various processes that are not practical with a roll-to-roll printing process. This increases the cost of the panels and decreases manufacturing throughput of the panels. For instance, a substrate with pre-formed channels in which the spheres ultimately reside is used. A paste conductor and the spheres that are not deposited in the channels must be scraped off the substrate, increasing the difficulty and cost of forming the panels.
Further, the processes of US patent application publication no. US2010/0167441 generally deposit pre-formed lenses over the diode array, where the shapes of the lenses are not optimized for the spheres and where the lenses are difficult to optimally position with respect to the spheres. Due to the large variations in indices of refraction between the silicon, lens, and air, there is significant reflection of light. U.S. Pat. No. 8,013,238 aligns lenses to millimeter sized spherical diodes with a vertical, elastomeric standoff, requiring the spheres be spaced in a square array, millimeters apart, which significantly decreases the active area of the photovoltaic panel. In the present application, Applicants disclose aligned lenses with a graded refractive index over a closed packed array of silicon microsphere diodes to reduce reflection of light from the silicon and allow for a more efficient PV panel. European patent application EP 1 586 121 B1 describes an antireflection coating for spherical PVs but the material deposition method is a vacuum process, so a continuous roll-to-roll process cannot be performed.
Other improvements over the processes of US patent application publication no. US2010/0167441 are also desirable, which improve the performance of the panels and simplify processing.
What is needed is an all atmospheric pressure technique to fabricate a highly efficient PV panel with an antireflective, graded index lens at a relatively low cost, using a roll-to-roll printing process.